A viral disease that kills wild and domesticated rabbits is spreading in the Western U.S.
Dr. Ralph Zimmerman, state veterinarian in New Mexico, said, “There are areas around the state where we’re not seeing rabbits at all.”
In April, his state became the first in the U.S. with confirmed infections in wildlife with rabbit hemorrhagic disease virus serotype 2, which affects the European rabbits raised as pets or farmed and at least some wild rabbits and hares native to North America. It may affect related species, such as pika.
The disease threatens wild ecosystems and a domestic rabbit industry valued by the U.S. Department of Agriculture at more than $2 billion, mostly in pet supplies and care.
USDA Animal and Plant Health Inspection Service spokesman Mike W. Stepien said May 27 that RHDV-2 had been detected this spring in domestic species, wildlife, or both in Arizona, California, Colorado, Nevada, New Mexico, and Texas. Among wild species, infections occurred in desert cottontail, mountain cottontail, black-tailed jackrabbits, and antelope jackrabbits. Study results also indicate Eastern cottontails are susceptible to the virus.
It also is spreading among domestic and wild rabbits in northern Mexico.
Unknown mortality rate
The World Organisation for Animal Health (OIE) indicates observed death rates for the virus have ranged from 5%-70%, with a mean mortality rate of 20% under experimental conditions. An April 2017 scientific article in the journal Veterinary Record indicates the death rate associated with RHDV-2 infections appeared to rise as the virus spread into Italy, citing two strains isolated in 2014 and 2015 that induced mortality rates of at least 80%.
A May 2020 scientific article in Veterinary Clinics of North America: Exotic Animal Practice indicates those rising death rates in Europe and recent study results suggest RHDV-2 has become more lethal. In one study, experimental infection of New Zealand white rabbits showed similar pathogenicity among RHDV-2 and RHDV strains, which kill 70%-90% of susceptible adult rabbits.
“Rapidly increasing fatality and infection rates suggest that RHDV-2 has evolved into a highly pathogenic calicivirus,” the article states.
Dr. Olivia A. Petritz is one of the report authors and an assistant professor of avian and exotic animal medicine at North Carolina State University College of Veterinary Medicine. She said estimating mortality rates for RHDV-2 in wildlife is difficult without good data on animal population sizes and infection status, which includes infected animals without clinical signs.
She thinks additional RHDV-2 outbreaks are likely in the U.S.
Classic RHDV strains are highly contagious and lethal to only one species, the European rabbit, according to the OIE. Those strains emerged worldwide in the 1980s and 1990s, and they since have caused limited outbreaks in the U.S. among domestic European rabbits.
RHDV-2, discovered in 2010 in France, infected and killed European rabbits, European brown hares, Sardinian Cape hares, and Italian hares as it spread across Europe. In Australia, where European rabbits and brown hares were introduced as game species, RHDV-2 arrived in 2015 and has since reduced rabbit populations in some areas up to 80%, the Veterinary Clinics of North America article states.
The virus reached North America by 2016, when animal health authorities confirmed infections on hobby farms in Quebec, APHIS information states. From February 2018 to March 2020, it caused sporadic outbreaks among pet and feral European rabbits in British Columbia, Washington state, Ohio, and in one New York City veterinary clinic.
APHIS identified infections among pet rabbits in New Mexico in March 2020, and the state saw die-offs among wild rabbits before APHIS confirmed in April that the disease had spread to wildlife.
Clinical signs, transmission
Rabbit hemorrhagic disease, whether from classical RHDV strains or RHDV-2, often kills rabbits without outward signs of disease.
“Many times, the only signs of the disease are sudden death and blood-stained noses caused by internal bleeding,” APHIS information states. “Infected rabbits may also develop a fever, be hesitant to eat, or show respiratory or nervous signs.”
California Department of Fish and Wildlife information states that the disease causes liver inflammation and prevents blood from clotting.
“Death is due to massive internal hemorrhaging and liver impairment,” CDFA information states.
RHDV-2 infection has an incubation period of three to five days, and animals may develop subacute to chronic disease, with lesions, lethargy, anorexia, weight loss, and jaundice, according to the OIE. Subclinical carriers may shed the virus for months.
Infected animals have developed gastrointestinal dilation, cardiac arrhythmias, heart murmurs, and neurologic abnormalities, OIE information states. The CDFA’s list of clinical signs also includes breathing difficulties, blue lips or mucous membranes, and bleeding from body cavities.
The virus spreads through contact with an infected animal or exposure to contaminated bodily fluids, hair, or carcasses, according to OIE information. Fomites and vectors—insect or animal—also spread the virus.
Contaminated rabbit meat may be one of the main methods of transmitting RHD to new countries, according to the OIE. How the disease entered the U.S. is unknown, according to APHIS.
APHIS officials are allowing the purchase of two killed-virus vaccines that are made in Europe and unlicensed in the U.S., but only under the direction of state animal health officials. The agency recommends using them only where the virus circulates in feral and wild rabbits.
The Washington State Department of Agriculture reported USDA officials had granted veterinarians permission to buy and distribute vaccine in their state, and Dr. Zimmerman hopes vaccination will help protect domestic herds in his state as well.
Local population effects
By May 21, about 480 domesticated rabbits in New Mexico had died of RHDV-2 infection, and another 500 had been depopulated, Dr. Zimmerman said. Most of those rabbits lived on farms, which ranged from a few dozen to hundreds of rabbits, although some were pets that likely became infected after their owners stepped in wild rabbit waste.
Dr. Zimmerman said his state stopped testing wild rabbits after confirming the disease was spreading in native populations, but the effects seem to vary by region. Members of the public were reporting more deaths among cottontails, which tend to live closer to people than do jackrabbits.
Kerry Mower, PhD, wildlife health specialist for the New Mexico Department of Game and Fish, said June 1 that the number of deaths had been high among jackrabbits and cottontails in affected areas, but the impact remained unknown. His department received reports of areas where rabbits disappeared while adjacent areas seemed to be unaffected.
“We do expect the species that depend on rabbits as a prey base will be affected, but we cannot predict the extent,” he said.
Rabbits are ubiquitous in New Mexico, although the state has not monitored or estimated population sizes.
Dr. Deana Clifford, senior wildlife veterinarian for the California Department of Fish and Wildlife, said a black-tailed jackrabbit found dead May 7 became California’s first confirmed case. It was among 10-20 dead jackrabbits found that week by a biological survey team working near Palm Springs in Riverside County.
In the subsequent two weeks, members of the public submitted about 35 reports of sick or dead rabbits, most in Riverside County and most describing a single dead rabbit, Dr. Clifford said.
“We are very early in the outbreak in California, so at this point we do not have a sense of how much mortality will occur,” Dr. Clifford said. “Large numbers of dead rabbits have been reported in other states, but there have also been continued sightings of live rabbits, so that provides some hope that some individuals will survive.”
The CDFW tested three more wild rabbits but found none positive for the virus, Dr. Clifford said. The state is focusing testing on counties neighboring Riverside County, areas of Riverside County that are distant from the first reported case, and species without known infections in the state, such as desert cottontail.
Dr. Clifford said the virus may have populationwide effects on rabbits, which are an important prey species. Her agency would need to conduct systematic surveys to estimate the mortality rate and potential population effects.
“The virus is very hardy in the environment, so we have focused on reducing the chances that human activities will spread the virus to new areas,” Dr. Clifford said. “We have a few small isolated rabbit populations in California, including the endangered riparian brush rabbit, so we are evaluating efforts we could do to protect that species.”
Dr. Zimmerman said in May that New Mexico authorities already were receiving increasing reports of coyotes in Albuquerque and other populated areas. He said the spread of the hemorrhagic disease raised concerns for the many species above rabbits in the food chain.
APHIS encourages veterinarians nationwide to watch for RHDV-2, report any suspicious illnesses or deaths to state and federal regulators, and submit samples for testing through the National Veterinary Services Laboratories’ Foreign Animal Disease Diagnostic Laboratory. RHDV-2 is a reportable disease in the U.S.
The agency was supporting diagnostic testing of wild rabbit carcasses, sharing information with state wildlife partners, and coordinating sample submissions and disease investigations.
Dr. Zimmerman urges biosecurity for anyone with rabbits, whether they own pets, show flocks, or meat-production herds. “Lock down and be cautious,” he said.